TOMICHI PORPHYRY COPPER-MOLYBDENUM DEPOSIT, GUNNISON COUNTY, COLORADO
The Tomichi Cu-Mo prospect is 50 km east of Gunnison, and 3-4 km northeast of the historic Tomichi Pb-Zn-Ag-Au district. The porphyry system was drilled between 1957 and 2012 by Climax, Cyprus, Cities Service, Molycorp, and Burnstone Ventures, with 52 core holes totaling 16,612 m. The property is currently under option to Zacapa Resources.
Cu-Mo mineralization is hosted within a ~36 Ma (K-Ar, whole-rock) granite porphyry complex at the south edge of the 35.8-35.47 Ma Mount Princeton batholith. Porphyry phases in sequence are fine porphyritic, medium/coarse, aplitic, seriate, and megacrystic, and all except the medium/coarse unit are associated with significant Cu and Mo. Weakly mineralized porphyritic microgranite is probably correlated with the 30-29 Ma Mount Antero Granite. Shell-shaped Mo and Cu zones are stacked, with Cu fringing and overlapping Mo, mostly in zones of potassic (biotite + K-feldspar) alteration. Unidirectional solidification textures near the carapaces of the aplitic and seriate units confirm the importance of these phases as sources of Cu and Mo. Veins include gray sericite ± biotite ± K-feldspar fracture halos (early-halo or ‘EH’ veins) with chalcopyrite and/or pyrite, ‘A’ granular quartz-sulfide veins with chalcopyrite and/or molybdenite, and ‘B’ drusy quartz veins with molybdenite. Postmineral normal movement along the ~45º west-dipping Copper Hill fault offset the Cu-Mo zones ~850 m down to the west, where considerable untested upside potential remains. Oxidation extends 40-60 m deep, with chrysocolla, malachite, cupriferous Mn oxides mixed with goethite, jarosite, and minor relict sulfides. Thin zones of supergene chalcocite replace chalcopyrite in some holes.
Inferred Resources (Canadian NI 43-101-compliant, open pit-constrained) are 711 M tonnes grading 0.21% Cu, 0.035% Mo, 0.017 g/t Au, 2.0 g/t Ag, and 0.216 ppm Re. The resource remains open at depth and to the north, east and southeast. Although Tomichi is commonly classified as a quartz monzonitic-granitic (low-F) porphyry Mo-Cu system, the reported Mo/Cu ratio is more similar to a Mo-rich porphyry Cu system such as Sierrita, Arizona and Brenda, British Columbia.